The introduction of high-density (HD) inks and corresponding stencil systems has created exciting design opportunities for garment screen printers. But achieving well-defined, three-dimensional garment graphics with these inks often proves more difficult than many printers expect. This discussion is designed to help printers improve their results with HD printing by addressing the difficulties most commonly experienced.

Note that this is a general guide to HD printing; I believe that there is never just one "right" way to do things. Every printer I have ever had the pleasure of talking to or working with has developed personal tricks and procedures to achieve the results they desire. These suggestions are ones that I've developed after nearly a decade of working with HD inks. My hope is that by reviewing my methods, other printers will be able to overcome problems they commonly experience with HD applications.

What defines a successful HD image?

Successfully printing high-density inks with maximum height and clean, sharp details requires a precise combination of artwork and separation technique, screen mesh, stencil thickness, ink viscosity, and printing technique. The better that all of these variables work together, the better the results you'll achieve, both in terms of quality and productivity.

HD artwork

In the ideal situation, the design is created with the HD-printing process in mind right from the beginning. Extremely fine detail is simplified or limited to areas where height is of secondary importance. Progressive layering of printing areas is used to increase the image height where needed. Registration difficulties are minimized by limiting shared edges between image elements, except where such edges add to the overall impact of the design. Image elements, such as negative areas and patterns, are used to increase the appearance of height. The highest areas of the print cover a limited portion of the overall image. Where larger raised areas are required, they are frequently broken up with a pattern.

In Figure 1, all of these design strategies have been utilized to create an attractive HD print that is comfortable to wear, has acceptable height, and is reasonably easy to produce at a profitable production rate. HD inks were part of the design plan when the original artwork was created. It is absolutely critical to the success of the job and the printing facility as a whole for artists to design with the limitations of the printing process and the facility's production capabilities in mind. In this case, the design resulted in a product that lent itself to efficient, high-volume production with good image quality and minimal waste prints. The best way to approach printing and production limitations in the artroom is to view them as design challenges rather than as confining restrictions. This will help keep the creativity of your artists flowing.

In contrast, adding in a special process like HD to a design that has already been created for conventional printing is seldom as successful as when the designer understands the limits of the HD process and then builds the design a-round them. Adding HD to an already created design will often force you to sacrifice something else in the print--perhaps another color. Most often, this sacrifice is necessary because there simply are not enough stations on the press to support HD in addition to other colors and flash-curing positions.

Separation technique

When separating art for HD printing, you should utilize a layering technique in almost all cases. In the print shown in Figure 1, the method of layering is fairly obvious. The separations were created so that the first layer is solid underneath the second layer, which is solid underneath the third. The first layer of this print is a light blue and is a solid shape. It looks much like an underprint but is not choked. The second layer is also light blue, but contains only the twill simulation lines and the outer edge where it prints under the navy blue. The last layer is just the outer edge of the navy blue. The stitch lines were knocked out of all three layers of the art. Although the stitch lines filled in a little on the base layers, it does not adversely affect the overall quality of the print, but rather adds to the realism of the simulation.

In general, this is the way all HD separations are set up. It should also be noted that the shared edges of each successive layer are the same size as or smaller than the previous layer. Some printers will choke each successive layer about a tenth of a point to make registration easier. This may help some people, but it is usually not necessary. Keep in mind that HD inks of the correct viscosity will not print properly without an even base layer for the upper layers to be printed upon.

Screens, stencils, and inks

The cornerstone of successful HD printing is matching the stencil thickness with the viscosity of the ink and the detail in the artwork (Figure 2). Very often I have seen printers attempting to force standard HD ink through a stencil that is too thick, usually in an effort to get adequate image height with only a single screen worth of ink. This is probably the most common mistake that printers make when they begin to work with HD ink.

Top-quality HD inks have a viscosity that has been designed to work with stencil thicknesses in the 200-micron range. The thicker viscosity and the very short body of the ink are what create the proper definition and the smooth, even surface, as well as that very squared-off, crisp-looking edge that is characteristic of all high-quality HD prints. These same properties make the ink extremely difficult to print through a stencil that is thicker than 200 microns.

With thicker stencils, a thinner ink will be needed in order to print properly. This may seem to be a perfectly logical way to easily get additional height from the ink. However, when more height is needed, it is much better to use a second screen and print an extra layer. You will enjoy faster setup time, better final print quality, and more efficient production using two screens with 200-micron stencils than a single, thicker stencil with ink that has been modified to print through it. More often than not, these thinner inks will not print with the qualities that people look for in an HD product. The edge will often be rough, the surface uneven, and the center of the print area will frequently have reduced height compared to the edges of the same print area.

In an effort to get a single screen to print with the height needed, a great deal of additional time will be needed at the press to try to correct these issues. This causes some unnecessary frustration on the part of the printers and can lead a shop to avoid HD printing altogether.

Usually, a 200-micron stencil is matched up with a mesh count of approximately 86 threads/in. Higher mesh counts are often matched with thinner stencils when the detail in the artwork requires it. Fine details are usually printed with 110-thread/in. meshes, and occasionally with even higher thread counts. Such higher mesh counts are generally matched with a stencil thickness of 150 microns or less. Mesh counts of 156 thread/in. with stencils that are 50 microns thick can be used for extremely fine detail.

For best success, all screens should be produced with good, low-elongation mesh that has a fairly thin thread diameter. Screen tensions should be maintained at 35-N/m or higher in order to ensure that the mesh will snap off and release the ink in a slow, smooth, and controlled manner. In general, more tension will be helpful, but it is not absolutely necessary.

The base layer of the design shown in Figure 3 was printed using an 86-thread/ in. mesh with a 200-micron stencil. If additional height were needed, then the same layer could be printed a second time with an identical screen (provided that the space were available on press). The next layer has much more detail. The angled lines are less than two points in width. Printing this level of detail properly required a 110-thread/in. mesh with a 150-micron stencil. The third layer of ink was printed through another 86-thread/in. screen with a 200-micron stencil. The negative detail in the stitches printed without difficulty; however the line width was expanded enough in the separation to allow the ink to spread a little without closing up. If the press being used is accurate and holds tight registration, this layer could also be printed a second time with an identical screen. Again, this would depend on the availability of extra stations on the press.

Printing technique

Standard squeegees with a durometer of 65, or 60/90/60 multidurometer blades, will work well in almost all HD applications. Usually, printing with a large (but not excessive) squeegee angle with a minimal amount of pressure is most effective. A slightly slower print speed will almost always help with print quality by minimizing the squeegee pressure needed. The squeegee should be sharp edged. Any kind of rounded or other special squeegee should be avoided. Excessive squeegee pressure will make it difficult, if not impossible, to print a clean and crisp squared-off edge.

Note that HD inks tend to be shear-thinning inks, so as the production run progresses, their viscosity reduces. Squeegee pressure should be gradually reduced to keep the amount of ink deposited consistent. Floodbar pressure will have little impact on the amount of ink deposited, and the floodbar will have little use beyond moving the ink into position over the mesh openings.

Off-contact should be slightly more than normal, and it should increase with each successive layer printed because the print surface is raised with each layer of ink that has been deposited. Usually, the first layer of HD ink will be printed with an off-contact equivalent to the height of two American nickel coins from the print surface. Smaller print areas and finer detail will require less off-contact. The off-contact will increase gradually with each additional layer printed and should grow by a distance slightly more than the deposit thickness of the previous layer.

Look for a gradual and smooth lifting of the screen from the print surface during the print stroke. Insufficient off-contact will make it difficult for the ink to release cleanly from the screen at the edges. It also can leave mesh marks in the ink. Too much off-contact will cause large areas of ink to remain in the stencil, rather than transfer to the garment. Printers will often increase squeegee pressure in an effort to correct both of these off-contact problems, which only leads to more problems.

All the print surfaces must be level in order to set off-contact properly. The quality and controllability of the press can make HD printing easier, and leveling of the platens should be part of your routine maintenance procedures. Individual adjustments of off-contact can be time consuming, but they are necessary for high-quality specialty printing. With practice, most printers become reasonably quick at this. Some inventive printers even have good success using things like homemade shims of varying thicknesses in the screen holders to simplify the off-contact adjustment process.

As with almost all types of specialty printing, press variables will change as the production run progresses. Flash times and temperatures can and should be decreased during the course of a run to help prevent excessive heat from building up on the press. Although each layer of ink will need to be flashed before the next is printed, HD inks have a very quick flash time. With the viscosity drop that HD inks experience on press, squeegee speeds can eventually be increased and, most importantly, squeegee pressure can be decreased.

The main point here, especially if you're using an automatic, is that you cannot walk away from an HD job and assume all conditions will remain the same during a long production run. Instead, you must continually monitor the press and printing results, making adjustments to compensate for heat and viscosity changes, increasing press speed, and protecting the quality of the prints. How much and when to adjust are skills the press operator will learn to master over time with each HD job he or she faces.

As mentioned previously, building HD layers sequentially is more effective than trying to print the entire desired thickness in a single pass. Consequently, your designs should strive to use every possible station on the press. The superior results and reduction in rejects you'll get by layering with multiple screens will quickly offset the costs of using the additional screens.

A final recommendation

One last word of advice to anyone who is beginning to work with HD or other specialty-printing techniques: Start with something simple, and make it work as well as possible before proceeding to more complex projects. Success with even a simple print provides an invaluable learning experience that you may never realize if you waste time struggling with an overly complex application.

About the author

Michael Beckman holds a bachelor of fine art degree in printmaking from Northern Arizona University, where he was the master printer for the school's visiting-artist program and an honors graduate. Involved in the screen-printing industry for more than 17 years, he has worked for many top printing operations in positions ranging from artist and art director to technical manager and general manager. Beckman serves as a technical consultant to the industry and is credited with the original development of high-density ink and printing processes, as well other specialty-decorating methods. He can be reached at MB Screen Printing Inc. in Portland, OR by e-mailing mbeckm1@attbi.com.

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